diff --git a/flake.nix b/flake.nix
index 32bd3e6..54a080a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -31,7 +31,7 @@
 
         stableToolchain = pkgs.rust-bin.stable.latest.default;
         stableToolchainWithRustAnalyzer = pkgs.rust-bin.stable.latest.default.override {
-          extensions = ["rust-src" "rust-analyzer"];
+          extensions = ["rust-src" "rust-analyzer" "llvm-tools"];
           # Extra targets if required
           # targets = [
           #   "x86_64-unknown-linux-gnu"
@@ -100,6 +100,7 @@
               cargo-nextest
               cargo-criterion
               cargo-outdated
+              cargo-llvm-cov
             ];
           });
       }
diff --git a/src/parser.rs b/src/parser.rs
index 0557435..f464bc7 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -342,6 +342,8 @@ fn color_test() {
     assert_eq!(c.1, Color::Rgb(255, 255, 255));
     let c = color(b"5;255").unwrap();
     assert_eq!(c.1, Color::Indexed(255));
+    let err = color(b"10;255");
+    assert_ne!(err, Ok(c));
 }
 
 #[test]